The present invention relates to a two-stroke internal combustion engine of the type that is suited for use in a portable power working machine and, in particular, to a two-stroke internal combustion engine that is capable of minimizing the quantity of so-called blow-by, i.e., the quantity of the air-fuel mixture that is discharged without being utilized for the combustion.
A conventional two-stroke internal combustion engine which is commonly used in a portable power working machine such as a chain saw includes an ignition plug disposed at the head portion of the cylinder, and an intake port, a scavenging port and an exhaust port, which are opened and closed by a piston, located in the trunk portion of the cylinder. With such a two-stroke internal combustion engine, one cycle of the engine is accomplished by two strokes of the pistonxe2x80x94there are no strokes exclusively assigned to the intake or exhaust.
More specifically, during the ascending stroke of the piston, an air-fuel mixture is inducted though the intake port into a crankcase below the piston. During the descending stroke, the air-fuel mixture in the crankcase is pre-compressed, producing a compressed gas mixture, which is then utilized during an initial part of the succeeding ascending stroke for exhausting the combustion gas from the exhaust port; i.e., the compressed gas mixture is blown into a combustion/actuating chamber, which is located above the piston at bottom dead center so as to expel the combustion gas toward the exhaust port. (Although the combustion/actuating chamber may be called a combustion chamber, an actuating chamber, a cylinder chamber, etc., those chambers are generically referred to in the present specification as xe2x80x9cthe combustion/actuating chamberxe2x80x9d). In other words, since the scavenging of the combustion gas is effected by making use of the gas flow of the air-fuel mixture, the unburned air-fuel mixture is more likely to be mixed with the combustion gas (exhaust gas), thereby increasing the quantity of air-fuel mixture that is discharged into the atmosphere without being utilized for the combustion. Because of the discharge of unburned fuel components into the atmosphere, two-stroke cycle internal combustion engines are not only inferior to four-stroke cycle engines in fuel consumption but also are disadvantageous in that a large amount of environmentally undesirable components, such as HC (unburned components in a fuel) and CO (incomplete combustion components in a fuel) are included in the exhaust gas, as compared with four-stroke cycle engines. Therefore, even if a two-stroke engine is small in capacity, the influence of these undesirable components on environmental contamination should not be disregarded.
With a view to minimizing these problems, there have been various proposals for the two-stroke internal combustion engine, the proposals being featured in that air is introduced into the combustion/actuating chamber prior to the introduction of air-fuel mixture so as to scavenge the combustion gas (see, for example, Japanese Patent Unexamined Publications H9-125966 and H5-33657). However, even with these proposals, it is difficult to sufficiently reduce the quantity of blow-by. Additionally, the layout and structure of the parts of the engine, including the air-fuel supply passageway and air passageway, are not sufficiently regulated, thus causing the engine to increase in size. Therefore, the two-stroke internal combustion engines proposed in these publications are still required to be further improved for the purpose of mounting them on a portable power working machine.
The present invention is directed to solving the aforementioned problems. It is, therefore, an object of the present invention to provide a two-stroke internal combustion engine that is capable of minimizing the quantity of the air-fuel mixture discharged without being utilized for combustion, of improving the fuel consumption and power of the engine, of reducing the content of poisonous components in the exhaust gas, and of rationally and compactly arranging the parts of the engine.
With a view to attaining the aforementioned objects, a two-stroke internal combustion engine, according to the present invention, has a cylinder, a piston received in the cylinder for reciprocating axial movement in ascending and descending directions and defining with the cylinder a combustion/actuating chamber, and a crankcase. A device is provided for producing an air-fuel mixture, and an air-fuel supply passageway is arranged to receive the air-fuel mixture from the air-fuel mixture producing device and supply it to the combustion/actuating chamber through an air-fuel mixture-feeding port opening to the combustion/actuating chamber. An exhaust passageway opens to the combustion/actuating chamber at an exhaust port, and a scavenging passageway communicates the crankcase with the combustion/actuating chamber and opens to the combustion/actuating chamber at a scavenging port. An air passageway is arranged for introducing ambient air into the scavenging passageway. With respect to the descending stroke of the piston, the exhaust port is positioned and configured to open before the scavenging port opens and the air-fuel mixture-feeding port is positioned and configured to open after the scavenging port opens. Therefore, air is introduced into the combustion/actuating chamber prior to the introduction of the air-fuel mixture into the combustion/actuating chamber.
In a preferred embodiment of the two-stroke internal combustion engine of the present invention, portions of the air passageway and the air-fuel supply passageway are arranged immediately adjacent each other and are provided, respectively, with a check valve. Advantageously, the portions of the air passageway and the air-fuel supply passageway are arranged one above the other axially of the engine.
It is further preferred that the air-fuel passageway and port are configured to expel the air-fuel mixture toward a combustion chamber at the top of the combustion/actuating chamber. In addition, it is desirable in a two-stroke internal combustion engine according to the present invention to provide a second air-fuel supply passageway arranged to receive the air-fuel mixture from the air-fuel mixture producing device and to discharge the air-fuel mixture into the crankcase.
In a desirable arrangement of a two-stroke internal combustion engine according to the present invention, the air-fuel mixture is produced by a carburetor that, which includes a portion of the air passageway and a portion of the air-fuel supply passageway, and each such portion has throttle valve, the respective throttle valves being interlocked with each other.
With the preferred embodiments of two-stroke internal combustion engine of the present invention as described above, external (ambient) air is inducted from the air passageway into the scavenging passageway and the crankcase upon each ascending stroke of the piston, so as to be stored therein at the end of the ascending stroke of the piston. At the same time, the air-fuel mixture is drawn in from the air-fuel-producing device into part of the crankcase so as to be stored therein.
When the air-fuel mixture inside the combustion/actuating chamber disposed above the piston is ignited and burns through the ignition thereof, the piston is caused to be pushed downwardly due to the generation of combustion gas. In the descending stroke of the piston, an exhaust port is opened first, and when the piston has further descended, the scavenging port formed at an upper end of the scavenging passageway is opened so as to allow the air which has been stored in the scavenging passageway and the crankcase and compressed by the piston is ejected from the scavenging port into the combustion/actuating chamber disposed above the piston, thereby allowing the combustion gas to be pushed toward the exhaust port by the air.
When the piston descends further after the scavenging port has been opened, the air-fuel mixture-feeding port is opened a moment after the scavenging port has been opened (for example, 10 degrees later in terms of the crank angle being suitable), thereby allowing a relatively condensed air-fuel mixture existing inside the air-fuel supply passageway to be expelled from the air-fuel mixture-feeding port into the combustion chamber at the top of the combustion/actuating chamber. The air-fuel mixture thus ejected is prevented from being mixed with the combustion gas due to the presence of an air layer that has been previously introduced therein in advance, thereby allowing the air-fuel mixture to revolve in the vicinity of the combustion chamber.
Thereafter, following the supply of air, a very lean air-fuel mixture comprising air and the air-fuel mixture is introduced from the scavenging port into the combustion/actuating chamber.
As mentioned above, since the air-fuel mixture-feeding port is opened a moment later than the scavenging port, thereby allowing a relatively condensed air-fuel mixture existing inside the air-fuel supply passageway to be blown out of the air-fuel mixture-feeding port toward the combustion chamber of the combustion/actuating chamber, the air-fuel mixture thus blown out is prevented from being mixed with the combustion gas due to the presence of an air layer that has been introduced therein in advance, thus enabling the air-fuel mixture to revolve in the vicinity of the combustion chamber. As a result, the quantity of blow-by, i.e., the quantity of air-fuel mixture discharged without being utilized for combustion can be reduced to as minimum as possible, and at the same time, the air-fuel mixture can be easily ignited, thus making it possible to improve the fuel consumption and to reduce the content of undesirable components in the exhaust gas.
Furthermore, the introduction of the condensed air-fuel mixture from the air-fuel mixture-feeding port to the combustion/actuating chamber can be always effected (even after the movement of the piston changes from the descending stroke to the ascending stroke) due to a difference in pressure between the combustion/actuating chamber and the air-fuel supply passageway, so that before the air-fuel mixture-feeding port is closed, the air-fuel mixture can be immediately drawn from the air-fuel mixture-feeding port into the combustion/actuating chamber through the air-fuel supply passageway (or the check valve thereof). As a result, the filling efficiency of the air-fuel mixture can be greatly improved, and at the same time, the power of the engine can be increased.
Furthermore, since portions of the air passageway and the air-fuel supply passageway are arranged immediately adjacent each other, the parts of engine can be rationally and compactly arranged, thus making it possible to easily mount the engine on a portable power working machine.